a) Field of the Invention
This invention relates to headphone units for use in active ear defender systems, as well as to headphone assemblies incorporating such headphone units and to active ear defender systems incorporating such headphone assemblies.
b) Description of the Prior Art
It is known to protect the ears of a person working in a noisy environment using a so-called active ear defender system, wherein the person wears a headphone assembly the drive units of which are driven with an electrical signal which is controlled to reduce to a minimum the instantaneous sound pressure within the cavity between the person's ear and the headphone drive unit. This is achieved by providing a microphone within that cavity, the microphone serving to sense the instantaneous sound pressure level within the cavity, the microphone output being used to control the electrical signal supplied to the headphone drive unit. Phase control of the headphone drive unit is most important, to prevent the system becoming unstable, and to this end the microphone is usually placed close to the drive unit, the physical size of which is maintained relatively small.
A known form of headphone for use in an active ear defender system as described above is disclosed in U.S. Pat. No. 4,644,581 (Bose Corporation). The headphone unit described in that Specification employs a drive unit of 23 mm diameter, and sound pressure levels in the headphone cavity of 125 dB at 300 Hz and 115 dB at 20 Hz can be achieved. However, field trials in various noisy areas have shown that the noise spectrum may extend over a greater frequency range, and that sound pressure levels as high as 140 dB are not uncommon, within the frequency range from 20 to 150 Hz. On account of the limited size of the drive unit in the arrangement of the U.S. Patent Specification referred to above, the headphone cannot be used to achieve sufficient sound pressure levels to give adequate defence against such noisy environments, especially if the frequency range is somewhat extended. Higher sound pressure levels could be achieved by reducing the volume between the drive unit and the ear cavity of the user, but then the passive performance of the headphone unit would suffer unacceptably.